…or would such a system be too unstable to last?
I don’t have a definitive answer, but it doesn’t seem immediately implausible to me.
Any extra moon of the Earth would have to orbit within Earth’s Hill sphere, which is the region of space where a body would be orbiting the Earth rather than co-orbiting the Sun alongside the Earth. The moon’s orbital radius is only about 25% of the radius of the Earth’s Hill sphere, so on some sense there’s “plenty of room” for another Moon to be orbiting at a larger (or smaller) radius.
It’s also plausible that the new moon would end up in an orbital resonance with the OG moon. This is the situation where the ratios of the orbital periods of the bodies is a ratio of small integers (2:1, 3:2, etc.) A satellite with a period of double the Moon’s period would be orbiting 23/2 ≈ 2.82 times farther out, still safely within the Hill sphere.
These orbital resonances can help stabilize orbits — or destabilize them, depending on the details (says Wikipedia). Unfortunately, I don’t know what those details are, which is why this isn’t a definitive answer.
The earth-moon system is more of a 2 planet system then a planet moon system. Adding a 3rd one as I understand it would require one of the 3 to orbit the other 2, more than as we conventionally think of moons orbiting the planet. I would think for such a system to work our current moon would have to move to a much closer orbit or much further, it’s currently in the middle so it may not be possible to have 3 stable orbits with those sizes.
This may be true if Earth + 2x Luna formed that way naturally; but the OP could be interpreted as “would the consequences of adding a second Luna sized moon” into the mix be (whether through divine action, or action of a divine-equivalent ET or future-advanced human technology)
In which case, what if we popped in a Luna-2 on the same plane and 180 degrees opposite to Luna-1?
Assuming that would be orbitally stable, what would the effects on Earth’s waves be, etc.?
And of course, “orbitally stable” would be with respect to the Earth - that the two Lunas wouldn’t fall towards each other or the Earth from their initial distances (present distance of Luna). The fact that the “Earth+Luna(s)” system as a unit would now have increased in mass, wouldn’t that result in the whole shebang either falling closer to the Sun or slowing down?
But this is impossible IRL because things already in motion/orbit don’t suddenly gain mass without an impact event, right?
Tides would have a third factor to them. Currently, the moon and the sun direct the tides, so we have stronger tides when they line up, weaker ones when they are at 90 degrees to eachother. A second moon would add to that, complicating tide charts, and if both moons and the sun were lined up, we’d have some pretty impressively high tides.
If you parked another moon in the same orbit as the current one, 180 degrees away, that would not be stable. Within a fairly short amount of time (which could be mere years, or could be thousands, but I doubt millions), the moons would collide.
They would have to be in substantially different orbits, and if you didn’t put them in an orbit that had a resonance, they would probably find one relatively quickly, or collide, or send one out of the system, or (unlikely I think) send one to collide with the Earth.
There’s no reason for it to do so. The mass that you are adding would also be in orbit around the Sun.
It wouldn’t take an impact event to increase the overall mass of the Earth/moon(s) system. With a bit of technology and a whole bunch of power, you could drag one of Jupiter’s moons into orbit.
The ISS doesn’t fall towards Earth when it docks with a transfer pod, does it? After all the transfer pod first has to match orbits with the ISS at which point docking shouldn’t impact anything.
I don’t currently have sandbox universe loaded, but I checked on YouTube and there are a few videos of people who made this work. Long term the system isn’t very stable, and eventually, bad things happen, so I guess it’s about how long you expect the system to last.
From what I understand, one of the hypotheses about the formation of our Moon is that when the accretion disk formed from the debris of the collision started to coalesce there were actually several moons that formed, which eventually became 2, and the second one basically merged with the current moon in a fairly low energy smash, coming up and merging from behind (this would be why one side of the moon is significantly thicker than the other if the theory is correct).
Anyway, I won’t link to the videos but you can watch them yourself or load sandbox universe if you want to check it out. I saw one guy who had a stable system for a couple of thousand simulated years, but eventually, every one I’ve seen here goes unstable eventually. If you make one of the moons significantly smaller you can get it to work a lot longer, perhaps indefinitely (for some definitions of that).
Put it in the Moon’s L4 or L5 location and it should be very long term stable. But note that this won’t work for another Luna-mass satellite. The third object (the second moon) has to be considerably less massive than the other two (Earth and the Moon).
IIRC, the critical ratio of the two smaller masses, for a Trojan situation, is about a dozen: That is, the more massive satellite needs to be about a dozen times as massive as the less massive one.
EDIT: To be fair, a dozenth the size of Luna would still make for quite a respectable satellite.
I assume you mean one-twelfth of the mass which would make it about 44% of the diameter. Yes that would be a respectably large moon.